Various types of containers from drinking cups to elongated canisters have been manufactured over the years with rolled brims about an upper periphery thereof. Such rolled brims or brim curls as they are often referred to in the art serve both structural as well as aesthetic functions which are critical to the acceptance of such containers by the consumer.
Initially, it is imperative that a consumer oriented product be aesthetically pleasing to the consumer both visually as well as functionally. That is, a drinking cup or canister having a sharp, bare upper edge would not be readily accepted by the consumer. Such a rim is not visually pleasing to the consumer nor is such a rim comfortable for the consumer during use. Further, such a container is not structurally sound and could readily collapse when handled by the consumer. Additionally, with canister type containers having lids placing thereon, not only may the lid readily slip off over a sharp, bare upper edge, the seal between the lid and canister is not reliable.
The rigidity of a particular container is effected by the tensile and bending stiffness in both the vertical and circumferential directions of the container. As noted hereinabove, one expedient for increasing the rigidity of a paper container is to form a brim about the top of the container. As is disclosed in U.S. Pat. No. 2,473,836 issued to Vixen et al., conventional brim curling mechanisms utilize complimentary curved dies in which the lower die is first moved upwardly around the upper end of the cup to the top edge of the cup where it firmly holds the cup top. The upper die is then moved downwardly to engage the uppermost edge of the cup between the dies with both of the dies then moving downwardly together to curl the upper edge of the container thereby forming a brim. This brim adds significantly to the rigidity of the overall cup structure.
U.S. Pat. No. 3,065,677 issued to Loeser discloses a similar brim curling mechanism for containers. A lower die having a curve forming upper surface is maintained stationary while an upper die having a curve forming lower surface descends downwardly toward the stationary lower die, deflecting the upper edge portion of the container secured by the lower die and again forming a brim about the upper periphery of the container. This brim, as stated previously, adds significantly to the overall rigidity of the container.
Containers of the above-mentioned type can be readily manufactured at relatively high speeds using conventional brim curl forming equipment by forcing an unfinished annular edge into a die which curls the brim outwardly forming a roughly elliptical toroidal rim. As is noted in U.S. Pat. No. 5,029,749, the orientation of the blank used in forming such containers may also aid in the manufacture of containers having brim curls formed thereon by reducing defects found in the brim curls. However, when manufacturing containers where either the paperboard stock material is relatively heavy and/or the radius of curvature of the annular edge to be rolled is relatively small, cracks often appear in the outer surface of the toroidal brim. Clearly, such cracks degrade the appearance of the rolled brim and can often degrade the functionality of the rolled brim particularly when the brim is to sealingly receive a lid thereon.
In an effort to overcome the above-noted shortcomings, pretreating at least the annular edge of the paperboard shells with steam has been introduced as exemplified in U.S. application Ser. No. 08/208,883 to Aloisi et al. and assigned to the assignee of the subject invention. Therein, the shells are housed in a steaming unit prior to their final formation such that at least the annular edge region is moistened which permits the brim curl to be more readily and reliably formed. While such a unit aids in the formation of the container, the cost of such a unit as well as the expense of operating and maintaining such a unit are an added expense to the overall cost of each container.
In addition, containers of the type discussed above often include outer polymer coatings, such as polyethylene, applied to the paperboard material. Such polymer coatings may be extruded onto the surface of the paperboard. It is imperative that the polymer coating bond to the paperboard material. To this end, one of three methods are generally used, namely, corona treatment, flame treatment, or polyethylene imine treatment. These treatments are not always sufficient, however, if the polymer coating or the paperboard is treated with additional coating materials. In these cases, peeling of the polymer coating from the paperboard material can occur which is highly undesirable. Such peeling is particular a problem for brim curled containers in view of the stress imposed to the upper portion of the container during formation of the brim curl.
In view of the foregoing, there is clearly a need for a container that can be reliably manufactured at high speeds which exhibits an annular edge having relatively small radius of curvature and/or which is formed of a relatively heavy paperboard material having a brim curl which is substantially defect free.